Experimental warming in boreal peatlands alters oribatid mite community composition and body size through peat drying

The ability of boreal peatlands to sequester large amounts of carbon is threatened under climate change, due to changes in both abiotic conditions and biological communities. Specifically, boreal peatlands are predicted to experience a temperature increase of ~8 °C, with a concomitant reduction in moisture levels, both of which are expected to directly affect below-ground communities such as microarthropods. Oribatid mites are one of the most dominant microarthropod communities in boreal peatlands; they contribute to carbon flux by feeding on decomposing organic matter and regulating microbial communities. Using ambient and experimentally warmed plots established for 7 years which led to warming-induced peat drying, we examined the response of oribatid mites to warming at two boreal peatlands that differ in vegetation (moss vs sedge) and water table (moisture). We hypothesised that warming may increase oribatid mite metabolic rates and reproduction which will increase juvenile proportion, total abundance, and diversity at the community level. We also hypothesised that warming would increase the abundance of the small-sized (asexual) oribatid mite species leading to a reduction in average community body size. At both study sites, warming altered community composition. Under warming at the moss-dominated site, there was a reduction in average community size driven by increased abundance of some smaller-sized, asexual species. At the sedge-dominated site, changes in community composition were driven by the loss of semi-aquatic species. Thus responses appear to be shaped by a combination of factors, including initial site conditions such as soil moisture, as well as seasonal variation and experimental treatments. • Warming effect was dependent on initial moisture level of the study sites. • Warming-induced drying was evident at the drier site after seven years of warming. • Oribatid mite community composition changed under experimental warming. • Community body size reduced with increases in some smaller asexual oribatid mites.